Hitherto, photo-curable compositions comprising an oligomer (prepolymer) having (meth)acrylate terminal, a reactive monomer and a photopolymerization initiator have been widely used in the fields of paints, inks, coatings, electric materials, image formation, etc. because of the advantages that they cure in a short time at room temperature without using solvents which are harmful to the human bodies.
The oligomer which is a main component of these photo-curable compositions includes representatively urethane (meth)acrylate, epoxy (meth)acrylate and polyester (meth)acrylate. Especially, urethane (meth)acrylate is noticed as paints and coating materials because of its superior impact resistance, heat resistance and ductility. For example, it has been proposed to cure urethane (meth)acrylate obtained using xylylene diisocyanate with active energy ray to form a film suitable for printing plates or reliefs. (cf. U.S. Pat. No. 3,907,865).
Further, a method for obtaining molded products by cell casting polymerization has also been proposed with noticing the excellent properties of urethane (meth)acrylate in addition to paints and coating materials. For example, there have been proposed a method comprising mixing a hydroxy-containing monomer with a polyisocyanate to allow urethanation reaction and radical polymerization to proceed simultaneously, thereby obtaining a transparent molded product (U.S. Pat. No. 3,553,174) and a method according to which a high refraction plastic lens superior in heat resistance and impact resistance is obtained using urethane resin obtained by radical polymerization of a reaction product of xylylene diisocyanate with a hydroxyl group-containing unsaturated compound (U.S. Pat. No. 4,443,588).
However, uses of the photo-curable composition of U.S. Pat. No. 3,907,865 are limited to coating fields represented by coating and painting and thickness of the coating film is very thin such as several .mu.m - several hundreds .mu.m. This is due to the fact that the irradiated light is absorbed and scattered by photo-polymerization initiators, powder, etc. to prevent the light from reaching inside. Thus, compositions curable to a deep portion of several mm--several cm have not yet been found. In an attempt to improve the deep curability, use of heat catalysts such as azobisisobutyronitrile, benzoyl peroxide, etc. has been proposed. However, when the resulting cure molded products are used for optical and electrical uses, there occur yellowing, optical strain and stress strain and these are significant problems especially for use as optical parts.
The methods of U.S. Pat. Nos. 3,553,174 and 4,443,588 can afford plastic lenses superior in heat resistance and impact resistance, but require a long time of 20-50 hours for polymerization molding after pouring into lens mold and thus productivity is very low. Such casting polymerization methods which require long time have significant problems in production of lenses for glasses for which optical characteristics are important and which must be produced in a wide variety. Thus, methods higher in productivity have been demanded. Another problem is that the raw material urethane (meth)acrylate is solid or highly viscous liquid at room temperature and suffer from the problem of inferior operability in casting polymerization.